Atherosclerosis is a disease of multiple etiology that is influenced by a variety of risk factors. Risk factors, however, can explain only a portion of the variability in this disease. The remaining unexplained variability is due, at least in part, to genetic factors mediated at the level of the arterial wall. The proposed studies will address potential genetically-mediated arterial wall factors by utilizing breeds of pigeons that are genetically susceptible (White Carneau, WC) or resistant (Show Racer, SR) to aortic atherosclerosis. Macrophages play an important role in the pathogenesis of atherosclerosis in pigeons, as they do in man. Although both WC and SR macrophages accumulate large amounts of cholesteryl esters when incubated with certain abnormal lipoproteins, there is no difference in the amount of cholesteryl esters that accumulate in macrophages from the two breeds. SR macrophages in culture, however, are able to efflux this excess cholesterol efficiently to an appropriate acceptor in the medium while, under the same conditions, WC macrophages are defective in cholesterol efflux. The purpose of the proposed studies is to define the cellular and molecular mechanism(s) responsible for this defect in cholesterol efflux and to test the hypothesis that susceptibility to atherosclerosis in the WC pigeon is mediated by an abnormality in cholesterol homeostasis in their macrophages resulting from a defect in cholesterol efflux. Three specific aims are proposed to test this hypothesis. Specific Aim 1 will determine the cellular and molecular mechanisms responsible for the lower rate of cholesterol efflux from cultured WC macrophages. Unlike most published studies, these studies will use macrophages in culture that are loaded with cholesteryl esters to levels found in macrophage foam cells from atherosclerotic plaques. This is accomplished by loading elicited pigeon peritoneal macrophages in vitro with cholesteryl esters or using macrophages from hypercholesterolemic pigeons that are already loaded with cholesteryl esters. Using a variety of agents to stimulate or inhibit potential regulatory steps in cellular cholesterol homeostasis, the proposed studies will determine how cholesterol efflux is regulated in pigeon macrophage foam cells, and the site of the defect in cholesterol efflux in WC macrophages. Specific Aim 2 will determine the extent to which individual variability in aortic atherosclerosis in WC pigeons is correlated with the defect in their macrophages to efflux cholesterol (expt. 1), and if differences in cholesterol efflux potential from macrophages cultured in vitro can be used to predict the subsequent severity of development of atherosclerosis (expt. 2). Specific Aim 3 will test directly the hypothesis that this defect in WC macrophages is responsible for their enhanced atherosclerosis. This will be done by transplanting macrophages from SR pigeons into WC pigeons and vice versa, and studying its effect on the extent and severity of experimental atherosclerosis.